Multi-radiator antennas are frequently used in for example cellular networks. Such multi-radiator antennas comprise a number of radiating antenna elements for example in the form of dipoles for sending or receiving signals, an antenna feeding network and an electrically conductive reflector. The antenna feeding network distributes the signal from a common coaxial connector to the radiators when the antenna is transmitting and combines the signals from the radiators and feeds them to the coaxial connector when receiving. A possible implementation of such a feeding network is shown in FIG. 1.
In such a network, if the splitters/combiners consist of just one junction between 3 different 50 ohm lines, impedance match would not be maintained, and the impedance seen from each port would be 25 ohm instead of 50 ohm. Therefore the splitter/combiner usually also includes an impedance transformation circuit which maintains 50 ohm impedance at the common port, i.e. the input port in case of a splitter and the output port in case of a combiner.
A person skilled in the art would recognize that the feeding network is fully reciprocal in the sense that transmission and reception can be treated in the same way, and, to simplify the description of this invention, only the transmission case is described below.
The antenna feeding network may comprise a plurality of parallel coaxial lines being substantially air filled, each coaxial line comprising a central inner conductor at least partly surrounded by an outer conductor with insulating air in between. The coaxial lines and the reflector may be formed integrally with each other. The splitting may be done via crossover connections between inner conductors of adjacent coaxial lines.
In order to preserve the characteristic impedance, the lines connecting to the crossover element include impedance matching structures.
The antenna feeding network is usually connectable to a coaxial feeder cable using a coaxial connector. The coaxial connector may be placed at the bottom or end plate of the antenna, which bottom plate is typically perpendicular to the coaxial lines. The body of the coaxial connector is typically attached to the bottom plate made of a conductive material such as metal. There are two major requirements for such a connector: firstly, impedance must be maintained and secondly, passive intermodulation (PIM) must be minimized. In order to meet these requirements, a consistent electrical connection between the coaxial connector and the coaxial line is required. The coaxial line inner conductor is usually soldered to the central pin of the connector, but attaching the connector body correctly to the antenna bottom plate or antenna body may be more difficult. In case a soft coaxial line, e.g. a PTFE cable, is attached to the connector, soldering the cable outer conductor, or shield, often results in PIM since all braids in the outer conductor are not correctly soldered. Also, the junction from the connector body to the antenna body or reflector, often via a bottom plate attached to the antenna body or reflector, can result in PIM. In the case of an antenna using air filled coaxial lines where the outer conductors of the coaxial lines are part of the antenna body or reflector, it is even more important to obtain a correct electrical connection between the connector body and the antenna bottom plate. This may be difficult to achieve in an antenna feeding network as described above, since the attachment of the coaxial connector to the bottom plate is subject to substantial mechanical forces from to the thick coaxial feeder cables connected thereto.
One solution to this problem is disclosed in WO2006006913, which shows an antenna where the coaxial connector is connected to the outer and inner conductors of a coaxial line using a separate coaxial cable (see FIG. 2). The coaxial connector is held in place mechanically by being attached to the bottom plate, but the electrical connection is provided by means of the separate coaxial cable. This solution may improve the electrical connection, but may be disadvantageous in other aspects. Firstly, the arrangement involves a large number of parts which may occupy valuable space in the antenna and may also result in high cost. Secondly, the separate coaxial cable may introduce losses. Thirdly, the connection may still suffer from PIM due to currents flowing from the body of the coaxial connector to the bottom plate and the outer conductor(s)/reflector.